1. Field of Invention
The present invention relates to technology for amplifying wireless frequency signals in mobile wireless communication devices such as cell phones that are small, lightweight, and low current consumption, and more particularly relates to technology for a wireless frequency power amplifier, a semiconductor device, and a wireless frequency power amplification method.
2. Description of Related Art
Mobile wireless communication devices such as cell phones have a wireless frequency power amplifier which enables amplification of high frequency signals. Output signals of the high frequency power amplifier are transmitted to a base station through the antenna of the mobile wireless communication devices. The high frequency power amplifier is required to be small, inexpensive, and low current consumption. High performance transistors such as GaAs-FET (gallium arsenide field-effect transistors), HBT (heterojunction bipolar transistors), and silicon germanium HBT (SiGe-HBT) enable more efficient high frequency amplification than other semiconductor devices.
The wireless frequency power amplifier is used to amplify the modulated high frequency signals to 100 to 1000 times higher output power level, and the amplified signals are transmitted through the antenna. The wireless frequency power amplifier consists of cascaded amplifies in order to obtain higher gain performance. In each of these cascaded amplifiers, high performance transistors as described above are used. The amplified signals at the each amplification stage of the cascaded amplifies are input to the next amplification stage, and output power level of the final stage amplifier reaches to approximately 1 W.
The output terminal of the high frequency power amplifier is terminated to the antenna. One end of the antenna is attached to the cell phone, but the other end is terminated to open space. If shielded materials which consist of metal or other conductors come close to the antenna, the high frequency power amplifier is mismatched because of the impedance fluctuation of the antenna. This fluctuation degrades the reliability of the high frequency power amplifier performance. This fluctuation can also cause the undesirable oscillations of the final amplification stage transistor, resulting in transistor failure due to excessive heating.
Higher linearity operation is required in the wireless frequency power amplifier used in CDMA (code division multiplex access), W-LAN (wireless local area network), and other types of digital modulation system that are commonly used in modern cell phones and wireless devices because of the amplitude and phase change contained in the modulated signals. The impedance fluctuation noted above affects the high linearity operation of the high frequency power amplifier and causes unstable operation of the high performance transistors in the high frequency power amplifier. The unstable operation of the high performance transistors causes the distortion of the high frequency power amplifier, resulting in the degradation of the transmission performance of the cell phones and wireless devices.
To avoid this problem, cell phones use an isolator, a circulator, or other components which have unilateral characteristics of signal transmission. By inserting such a unilateral component between the wireless frequency power amplifier and the antenna to isolate the output terminal of the wireless frequency power amplifier from the input terminal of the antenna, signals do not return from the antenna to the wireless frequency power amplifier, and impedance fluctuation at the antenna is not observed at the output terminal of the wireless frequency power amplifier. However, isolators, circulators, and other such unilateral components include a magnet, ferrite, or other such material, imposing a significant limitation on the size and weight reduction and degree of integration that can be achieved. With priority given to reduced size and price, unilateral components such as noted above are increasingly omitted from newer cell phones.
FIG. 12 shows an example of the transmission/reception circuit unit for the wireless frequency that is used in a W-CDMA (Wideband-CDMA) cell phone. Transmission from the modulation/demodulation block passes a filter 90 and is input to the wireless frequency power amplifier 92. An isolator 93 is connected to the output node of the wireless frequency power amplifier 92. The isolator 93 is connected to the antenna 95 through a wireless frequency switch 94.
FIG. 13 shows a power amplifier protection circuit described in U.S. Pat. No. 6,278,328 B1 (corresponding to Japanese Laid-open Patent Publication No. 2000-341052). When a greater than predetermined voltage is applied to the collector of the last stage transistor, this protection circuit supplies a feedback current through a device disposed between the base and collector of the final transistor, and can thereby protect the wireless frequency power amplifier without using an isolator.
FIG. 14 shows a wireless communication system with electronic components for wireless frequency amplification as taught in U.S. Patent Application Publication No. 2004/0135633 A1 (corresponding to Japanese Laid-open Patent Publication No. 2004-140633). This system has a capacitance device inserted between the output node of the transistor used in the last stage for wireless frequency signal amplification, and the gate node of the transistor in the current mirror circuit of the output level detection circuit ODT, and reflects variation in the output power accompanying fluctuation in the antenna impedance in the detection current of the output level detection circuit.
Japanese Laid-open Patent Publication No. 2005-045471 teaches connecting a separate detection circuit to the input node and output node of the power amplifier circuit, and comparing the outputs of the two detection circuits with a comparator to detect the gain fluctuation. By controlling the gain of the amplification circuit based on this gain fluctuation, the linearity of the amplification circuit can be improved.
One conventional technology for rendering a W-CDMA cell phone is to incorporate a wireless frequency power amplifier in the transmitter/receiver circuit unit rendered as shown in FIG. 12. This arrangement does not have a protection circuit for protection against impedance fluctuations in the antenna 95 incorporated in the wireless frequency power amplifier 92. The isolator 93 must be connected in order to maintain the transmission quality of the cell phone and to enable stable operation. However, because the isolator 93 includes ferrite or a permanent magnet, for example, the isolator 93 interferes not only with reducing the size but also reducing the weight of the cell phone or other mobile communication device.
Electrically, loss in the forward transmittance characteristic is preferably 0 dB, but in reality loss is typically from 0.5 dB to 1.0 dB. The isolator 93 is inserted between the wireless frequency power amplifier 92 and the antenna 95, and this makes it necessary to increase the output level of the wireless frequency power amplifier 92 in order to compensate for power consumption by the isolator 93. This is a factor increasing the power consumption of the wireless frequency power amplifier, which either shortens the connection time of the cell phone or requires increasing the battery capacity.
The power amplifier protection circuit taught in U.S. Pat. No. 6,278,328 B1 was proposed to solve the foregoing problem (see FIG. 13). When the load terminated at the output terminal of the wireless frequency power amplifier fluctuates, a voltage exceeding a predetermined threshold level applied to the collector tries to activate the protection circuit. Technology including this protection circuit is, however, referenced for application in the second-generation digital cell phone network system standardized in Europe, commonly known as the Global System for Mobile Communications (GSM).
An alternative technology of the related art is taught in U.S. Patent Application Publication No. 2004/0135633 A1. This technology disposes a current mirror circuit to the transistor in the final stage, and tries to protect the wireless frequency power amplifier by detecting the current variations that occur in conjunction with antenna impedance fluctuations (see FIG. 14). This technology is only referenced in the GSM system and the similar Digital Cellular System (DCS).
In addition, while both disclosures address protecting the wireless frequency power amplifier from damage due to load variation, they are silent about the adverse effects on adjacent channels caused by degraded modulation precision in the modulation signals and increased signal distortion, and otherwise compensating for degraded transmission quality.
Amplitude variation is not included in the output modulation signal with the modulation methods that are used in the GSM and DCS systems. With the technologies taught in the above-noted examples of the related art, a change from a state in which the antenna is normally terminated to a state in which the load fluctuates can be detected by monitoring change in the power, voltage, or current of the wireless frequency signal at the collector node of the final-stage signal transistor. However, with digital modulation systems, such as W-CDMA, CDMA, PDC (personal digital cellular), EDGE (enhanced data GSM environment), and WLAN, the modulated signal always includes amplitude change in the signal level, and the technologies cited above cannot differentiate amplitude change in the modulated signal and amplitude change caused by load fluctuation.
FIG. 15, FIG. 16A, FIG. 16B, FIG. 17A, and FIG. 17B show the input/output characteristic of a prior art wireless frequency power amplifier with 50-Ω termination. As shown in FIG. 15, FIG. 16A, and FIG. 16B, the current amplitude waveform at the collector (FIG. 16B) and the voltage amplitude waveform applied to the collector (FIG. 16A) are constant in the saturation range because the output power is constant for the input signal level. However, as shown in FIG. 15, FIG. 17A, and FIG. 17B, the output power changes according to the power level of the input signal in the unsaturated range, and the current amplitude waveform to the collector (FIG. 17B) and the voltage amplitude waveform applied to the collector (FIG. 17A) therefore also change.
Amplitude variation is not contained in the modulation signal in the GSM and DCS methods, and the wireless frequency power amplifier can therefore be operated in the saturation range. In addition, because there is no amplitude variation in the input signal, the output power is constant, and the current amplitude and voltage amplitude of the collector are also stable at a constant level. If the antenna impedance fluctuates and the operating state of the wireless frequency power amplifier changes under these conditions, the amount of the change of the observed output power, current amplitude, and voltage amplitude at the terminal of the wireless frequency power amplifier can also be detected as due to load fluctuation.
In a digital modulation system such as W-CDMA that uses the wireless frequency power amplifier in the unsaturated region, the input modulation signal includes amplitude variation as the modulated signals even during normal operation with 50-Ω termination. The output power, current amplitude, and voltage amplitude are therefore always changing and never constant. As a result, when the antenna impedance changes and the operating state of the wireless frequency power amplifier changes, variation caused by load fluctuations cannot be electrically detected from the change in the output power, current amplitude, and voltage amplitude detected at the collector node of the transistor in the last stage.
The arrangement taught in Japanese Laid-open Patent Publication No. 2005-045471 detects change in gain from the input/output terminals of the power amplification circuit. The gain of the power amplification circuit cannot be accurately detected, however, if there is a nearby metal shield, for example, that causes the terminal impedance of the antenna connected to the output terminal of the power amplification circuit to vary. As a result, not only can linearity not be improved, but the gain of the preceding stage increases if the power detected at the output terminal decreases because of the variation in the terminal impedance of the antenna, and the transistor in the preceding stage may fail. Furthermore, because the gain of the input/output terminals of the power amplification circuit is set high, normally about 30-40 dB, detection error increases in the gain detected from the input/output terminals, and there is insufficient improvement in linearity.